Previous studies suggest that use of the recommended prosthetic foot stiffness category and/or ankle power output setting may not optimize the function of Veterans with transtibial amputations (TTAs). Due to the functional impairments and lifetime of healthcare costs incurred by Veterans with TTAs, it is vital to determine how different prosthetic foot and ankle properties/parameters such as passive-elastic prosthetic foot stiffness and battery-powered prosthetic ankle power output affect the metabolic cost and biomechanics of Veterans with TTAs. Further, determining the optimal prosthetic foot stiffness and/or power output could dramatically improve the function of people with TTAs, which would promote increased physical activity, while reducing pain and the risk of comorbidities. Veterans with TTAs typically use a passive-elastic prosthetic foot such as the ssur Low Profile Vari-flex with Evo (LP) with a stiffness category based on manufacturer-recommendations, user body mass, and anticipated activity level. Previous evidence demonstrates that Veterans with TTAs using the recommended passive-elastic prosthetic foot stiffness category for walking have greater metabolic demands, asymmetric biomechanics, leg and back pain, and discomfort compared to non-amputees. Further, some Veterans use a battery-powered ankle-foot prosthesis such as the Ottobock BiOM. [The ssur Low Profile Vari-flex with Evo (LP) is the only passive-elastic prosthetic foot that integrates with the BiOM. The LP is utilized beneath (in-series with) the BiOM with a stiffness category based on manufacturer-recommendations, user body mass and activity level. Then, the BiOM is tuned by adjusting nine different parameters within the prosthesis using different software settings so that the combination of the user and prosthesis produce net positive prosthetic ankle work equivalent to average non-amputee net positive ankle work within two standard deviations (SD) of the mean. Use of BiOM has resulted in normative metabolic costs and biomechanics for people with TTAs compared to non-amputees, but one study found that use of the BiOM with a recommended LP prosthetic foot stiffness category and prosthetic power output setting did not elicit differences in metabolic cost at preferred walking speeds compared to use of participants? own passive-elastic prosthesis.] We will systematically assess how prosthetic foot stiffness and power output affect physical function in Veterans with TTAs. First, we will quantify prosthetic and torsional stiffness (force/displacement or torque/angle) values of LP prostheses. Determining stiffness values (in kN/m and kNm/rad) for a given passive-elastic prosthetic foot stiffness category is essential for deriving the dynamic biomechanical function of prosthetic feet. Then, we will systematically measure and compare the metabolic cost, biomechanics, muscle activity, and satisfaction resulting from using the LP passive-elastic prosthetic foot alone with four stiffness categories, and the BiOM battery-powered ankle-foot prosthesis combined with four LP prosthetic foot stiffness categories and three different magnitudes of prosthetic ankle power output during walking over a range of speeds. Our results will provide a mechanistic understanding of prosthetic stiffness and power to inform prosthetic design and control, which will improve and expedite rehabilitation of Veterans with TTAs and the quality of services provided by the VA, while saving time, money, and resources. By improving prosthetic design and control, our results could ultimately allow Veterans with TTAs to regain the greatest possible level of physical function, improve their health, maximize their recovery and rehabilitation, facilitate their return to work/duty, and potentially improve their quality of life. Long-term goals for prosthetic design should include ?smarter? prostheses that can change stiffness dynamically, generate net positive mechanical work, and dissipate mechanical work. However, a critical barrier to developing such advanced prostheses is our lack of a fundamental understanding of the effects of different prosthetic foot stiffness and power output on the metabolic costs, biomechanics, and satisfaction of Veterans with TTAs during level-ground walking.